The present invention relates to improved drying techniques for materials which are difficult to dry. In particular, the present invention relates to improved drying techniques which extend the use of agitated pan dryers or the like to drying of difficult to dry chemical materials.
In the production of numerous chemical products, including pharmaceuticals, it may be necessary to dry intermediates and/or the final product. This is usually accomplished by means of a drying apparatus, of which there are many different types, and which may be classified according to the drying operation. For example, Perry's Chemical Engineers Handbook (5th Edition) separates dryers into three categories; direct dryers, infrared or radiant-heat dryers, and indirect dryers.
Agitated pan dryers are generally included in the classification of indirect batch dryers wherein the heat for drying is transferred to the wet solid through a retaining wall. Liquid which is vaporized from the wet solid is removed by means separate from the heating means. In general, the rate of drying depends on the contact between the wet solid and the hot surfaces of the dryer.
Standard agitated pan dryers as shown in schematic cross-section in FIG. 1, consist of a relatively shallow flat-bottomed pan 10, covered by a dished or conical cover 20. The bottom and walls of the pan 10, are surrounded by a jacket 30, to contain the heating medium, such as steam. However, it is noted that not all agitated pan dryers include a jacket for the heating medium. A central vertical shaft 40, attached to a drive means 42, carries a slow-moving, heavy-duty agitator 45, which stirs the material in the dryer and moves the material toward and away from the heat-transfer surfaces. The agitator shaft 40, may enter either through the cover 20, or through the bottom 15, of the pan 10, and may include additional means to scrape the heat-transfer surface or to better agitate the material during drying. Heating medium may also be circulated within the agitator to add extra heating surfaces, or the only heating surfaces when no jacket is provided. The blades of the agitator may be capable of being raised and lowered to accommodate different loads within the dryer, and to adjust to the changing level of the product during drying. Agitated pan dryers may be operated either under atmospheric pressure or under vacuum. In both cases, the cover is normally provided with an outlet 50, for the release of vaporized liquids, the outlet 50, being attached to a vacuum connection if desired. A charge/discharge port 60, for charging wet material and removing dried material is normally provided through the side of the pan 10, but may also be provided through the bottom 15, of the pan 10. In the alternative, no charge/discharge port may be included and material may simply be charged and withdrawn by opening the cover 20.
Agitated pan dryers are most useful for drying batches of material which must be agitated during drying, e.g. materials which are hard to handle or for which continuous drying would be uneconomical. Agitated pan dryers are particularly useful when solvents are to be recovered upon vaporization from the wet solid; or when drying must be done under high vacuum. However, agitated pan dryers are not generally suitable for materials which suffer particle degradation during drying or which form into balls and caseharden during drying.
Many chemical products, especially pharmaceutical products, are organic in nature and may decompose if exposed to excessive temperatures. Further, such products may not be crystalline in nature, may have very small particle sizes, and may require removal of toxic, and/or flammable solvents. Volatile content following drying is often required to be very low, e.g. less than one percent. Such products may be very difficult to dry. In particular, the material often becomes sticky during drying and may form into balls which can easily caseharden. When this occurs, the required low volatile content can not be met. Therefore, long drying cycles are often necessary to obtain satisfactory results.
Direct drying wherein there is a direct contact between the wet solid and drying medium, such as hot gases, is also known. In such dryers the vaporized liquid from the wet solid is normally carried away by the drying medium. Direct dryers are often referred to as convective dryers. Most standard convective dryers can not be easily operated under vacuum and therefore may not be applicable to the drying of chemical or pharmaceutical products in which toxic solvents must be removed.
To increase drying efficiency, direct drying means have previously been added to indirect dryers. In particular, agitated pay dryers may be converted to include means for blowing drying medium over the surface of the wet solid in addition to the standard indirect heating though the walls and bottom of the pan. This technique helps to extend the range of types of materials that can be dried efficiently in agitated pan type dryers. One such combination is described below.
FIG. 2 is a cross-sectional view of a nutsche type filter/dryer, generally designated by reference numeral 100, as known in the prior art. In particular the nutsche filter/dryer 100, is a standard nutsche type filter which has been modified for use as a dryer and is essentially a variant of an agitated pan dryer. The similarities will be evident from the following description. In particular, the nutsche filter/dryer 100, includes a compression vessel 120; a gas inlet 130; and a gas outlet 140, having a dust collector 145, connected thereto. The nutsche filter/dryer 100, also includes a drive means 150, connected to a main shaft 154, having two sets of extending arms mounted at 90.degree. to each other. A first set of arms comprise flat blades (not shown) which act to smooth product 190, introduced to the vessel 120, in batches suitable for drying. A second set of arms 156, include multiple agitator extensions 158. The filter/dryer 100, further includes an inner discharge tube 160, situated within an outer discharge shaft 170, and a filter plate 180, located at the base of the vessel 120.
The main shaft 154, may be designed to both rotate and move vertically within the vessel 120. The first set of arms are fixed to and carried by the main shaft 154, while the agitator arms 156, can be moved vertically and independently of the main shaft 154. The inner discharge tube 160, is designed to move vertically within the fixed outer discharge shaft 170.
In use, the inner discharge tube 160, and main shaft 154, are raised to their highest vertical position. This in turn raises both sets of arms to their highest position. A feed slurry of product 190, to be dried is fed into the space bounded by the filter plate 180, the walls of the vessel 120, and the discharge tube 160. Because the filter plate 180, occupies the space which would normally be occupied by the heated plate of an agitated pan dryer, heating medium is circulated through the two sets of arms (i.e. the flat blades and the agitator arms 156). In this manner heat is transferred to the product 190, to evaporate solvent therefrom. Roughly sixty percent of the heat transfer is accomplished through the agitator arms 156, and flat blades, with the remainder being accomplished by contact between the heated walls of the vessel 120, and the product 190.
In order to increase drying times and efficiency, recirculated nitrogen gas may be fed into the vessel 120, to cause some direct or convective drying to occur. Attempts to circulate the nitrogen gas either up through the filter plate 180 or down through the product 190, may be largely frustrated and ineffective when the product 190, is difficult to dry for those reasons given above; e.g. small particle size, sticky, likely to caseharden, etc. This is because such product 190 either plugs the filter plate 180, or effectively seals off the nitrogen gas flow. Therefore, the nitrogen gas may be simply fed through the inlet tube 130, to pass over the surface of the product 190, as illustrated by the arrows within the vessel 120. The drying gas exits through the gas outlet 140, and the dust collector 145, and then is recirculated for further use. As the drying gas passes over the product 190, limited convective drying occurs and volatiles within the product 190, are evaporated. This is known as cross flow drying.
Perlmutter describes a classic drying curve, as shown in FIG. 3, wherein a constant drying rate takes place during a first phase. According to Perlmutter, the constant rate period is governed by external factors such as the gas mass velocity and thermodynamic state as well as the physical state of the product. Perlmutter particularly notes that when drying products having a tendency to form balls in the constant rate drying phase, convection drying should be carried out on a static (non-agitated) bed and should so continue until the critical moisture content is reached. Perlmutter further suggests that during the falling rate period, cake properties and heat input are the controlling factors. Finally, in the diffusion period, the agitator arms break up product clumps to provide a final product which is homogenous and fine powder. (See Perlmutter; Principles Of Pressure Nutsche Filter-Dryer Technology; Drying '92; edited by A. S. Mujumdar; pp 1321-1329; Elsevier Science Publishers, B. V.; 1992).
However, as will be explained below, drying in real world applications has proven to be more complicated than suggested by the classical theory. For example, using the same gas mass flow rates, two dryers may exhibit dramatically different drying performances. Moreover, it has been found that even doubling the flow rate of nitrogen gas provides only marginal improvement and actually significantly worsens thermal efficiency, in spite of the opposite conclusions which would be drawn from the classical theory.
Therefore, there remains a need in the art for improvements to convection drying of chemical compounds in agitated pan type dryers.